Insert Into has a varchar(max) issue

This type of  'Insert Into'  has been working fine for me.
But, I have run into a situation where @sql is larger than 8000 characters.
I thought that if I declare @sql as varchar(nomax) I should get over 2 gb of space.  
It seems to use 8000 not the larger 2 gb for MAX.

 DECLARE @sql VARCHAR(max)

SET @sql = N'INSERT INTO [#IssuesDetails]
            SELECT  * from openquery
            ([GETPAID],    '''
                      + Replace(@sql, '''', '''''') + ''')'

      EXEC (@sql)      

I attached the error.
I can do a work around but wondering why this does not work.
It seems to ignore the NOMAX.
When I run the SSRS report with less parameter choices everything works fine.
This 'Insert Into' is reading records from Oracle tables and inserting them into a SQL table called #IssuesDetails
NomaxError.PNG